A panel of purine monoclones have been developed that recognize differentiation antigens on human T lymphocytes. Several of these reagents have been developed that identify functionally distinct subpopulations of T cells. Some of these are similar to those described by other laboratories. One unique reagent, clone 9.3, reacts with all helper/inducer T cells and all cytotoxic T cells, as well as their precursors, but not with suppressor T cells. Clinical studies are under way, using these reagents to characterize the alterations occurring in lymphocyte subpopulations of patients with acute and chronic GVH disease. Additional studies are being performed to establish methods and to test the feasibility of using murine monoclonal anti-T cell antibodies for the prophylaxis and treatment of GVH disease. Our basic investigations now focus on a biochemical analysis of the surface molecules identified by these anti-T cell antibodies. Currently, we are analyzing the surface molecule responsible for spontaneous E-rosette formation. This molecule has an estimated molecular weight of approximately 50,000 (and is referred to as Tp50). When anti-Tp50 antibodies bind to the cell surface, several T-cell functions, including lymphocyte transformation in response to mitogens, antigens and allogenic cells are blocked, and the lytic activity of cytotoxic T cells and NK cells is blocked.